In various parts of the world, data of text broadcastings are transmitted utilizing the vertical blanking periods of video signals. FIG. 28 is a table showing the types of text broadcastings employed in typical areas, the superimposition lines on which the text broadcastings are superimposed, and the transmission clocks.
In order to sample text data of these text broadcastings, a broadcast text data sampling apparatus as shown in FIG. 27 has conventionally been used. Hereinafter, the construction and operation of the conventional text data sampling apparatus will be described with reference to FIG. 27.
In FIG. 27, an analog video signal, in which text data is superimposed on the vertical blanking period, is inputted through a video signal input terminal 2001. The analog video signal is sampled with a sampling clock fs(MHz) by an A/D converter 2002 to be converted into a digital signal. The digital signal is inputted to a low-pass filter 2003 which performs noise removal or the like, and to a synchronous separation circuit 2004 for separating a vertical sync signal VSYNC and a horizontal sync signal HSYNC from the video signal. In a slice level calculation circuit 2005, an optimum slice level SL is calculated from text data in a predetermined line and a predetermined position, on the basis of the vertical sync signal VSYNC and the horizontal sync signal HSYNC which are outputted from the synchronous separation circuit 2004. In a binarization circuit 2006, the signal which has been subjected to noise removal by the low-pass filter 2003 is converted into a binary signal of 0 or 1 with the slice level SL calculated by the slice level calculation circuit 2005. In the vertical blanking period, there is a period called “clock run-in” for synchronization between the broadcast text signal and the sampling clock. A PLL circuit 2007 performs synchronization of the sampling clock having the same cycle as that of the transmission clock, to the clock run-in, under control of a controller 2008, thereby generating a sampling clock in synchronization with the clock run-in. In a sampling circuit 2009, using the sampling clock so generated, text data are sampled from the binary signal. Then, in a decoding circuit 2010, the sampled data are subjected to a decoding process including error correction or the like, according to the type of the text broadcasting. The data decoded by the decoding circuit 2010 are transmitted to a display circuit (not shown) through an output terminal 2011, and display according to the type of the text broadcasting is carried out.
However, the conventional sampling of text data by using a sampling clock synchronized with the clock run-in by the PLL circuit, has the following drawbacks.
(1) When phase error occurs in the signal in the middle of the line due to group delay or the like, phase error occurs in the text data, resulting in sampling error.
(2) In order to suppress such group delay, a multi-tap filter, i.e., a higher-order filter for waveform equalization, is required. However, since such filter is large in circuit scale, the circuit scale of the whole apparatus is undesirably increased.
(3) Although sampling of text data is carried out at a cycle equal to the cycle of the transmission clock of the text signal, since the transmission clock varies with the type of text broadcasting, a sampling apparatus provided with a PLL circuit having a specific clock for each type of text broadcasting is required. Therefore, it is difficult to deal with plural types of text broadcastings with a single sampling apparatus. Further, it is difficult to fabricate a PLL circuit capable of outputting wide-range and stable clocks and, therefore, it is difficult to deal with plural types of text broadcastings having different transmission clocks, with a sampling apparatus provided with one PLL circuit.
(4) When the signal is degraded due to distortion of the transmission system or the like and thereby the data in the clock run-in have gone, the sampling clock cannot be synchronized with the clock run-in, resulting in sampling error.